A Post-it note (or sticky note) is a small piece of paper with a re-adherable glue strip on its back, made for temporarily attaching notes to documents and other surfaces. A low-tack pressure-sensitive adhesive allows the notes to be easily attached, removed and even re-posted elsewhere without leaving residue. The Post-it's signature adhesive was discovered accidentally by a scientist at 3M. Originally small yellow squares, Post-it Notes, and related products are available in various colors, shapes, sizes, and adhesive strengths.

Source: Wikipedia